In a known example of this type of pulse wave detection apparatus, a sensor chip, on which a pressure sensor (strain sensor) array has been formed using MEMS (Micro Electro Mechanical Systems) technology, is pressed against a measurement site through which an artery passes, and change in the internal pressure of the artery, that is to say a pulse wave, is measured in a noninvasive manner using tonometry, as disclosed in Patent Literature 1 (JP 2011-239840A) for example.
Among commercial products for measuring blood pressure (including change in the internal pressure of an artery) in a noninvasive manner using tonometry, there are products that include a sensor chip having the pattern layout shown in FIG. 16(A). In this example, the sensor chip 101 includes an approximately flat plate-shaped silicon substrate 102 that is elongated in one direction (X direction), a pressure sensor array 110 formed on the silicon substrate 102 at approximately the center in the Y direction (direction perpendicular to the X direction), and electrode terminal arrays 120A and 120B formed along long sides 102a and 102b on two sides in the Y direction. The pressure sensor array 110 is made up of pressure sensor (strain sensor) elements 11 arranged side-by-side in the X direction with a fixed pitch. The electrode terminal arrays 120A and 120B are each made up of gold bump electrodes 121, 121, . . . arranged side-by-side in the X direction with a fixed pitch. The pressure sensor elements 11l of the pressure sensor array 110 are connected to the corresponding gold bump electrodes 121 of the electrode terminal arrays 120A and 120B via interconnects (not shown) formed on the silicon substrate 102. As shown in FIG. 16(B) (cross-section along line B-B in FIG. 16(A)), the pressure sensor array 110 is formed on a thin region 119 provided by forming a recessed shape in the lower surface side (−Z side) of the silicon substrate 102. The lower surface side of the silicon substrate 102 is supported by a glass plate 103. Through-holes 103A and 103B, which allow the flow of air between the thin region (recessed portion) 119 and the outside, are provided in the glass plate 103 so as to not hinder flexure of the pressure sensor elements 111 in the Z direction.
As shown in FIG. 17, in a product with the sensor chip 101 mounted thereon, flexible wiring boards 130A and 130B are connected to the gold bump electrodes 121, 121 of the electrode terminal arrays 120A and 120B. Processing circuits (not shown) for processing signals from the pressure sensor elements 111 are implemented on the flexible wiring boards 130A and 130B. Also, a protective sheet 140 made of resin is provided so as to cover the entirety of the upper surface of the sensor chip 101.
When performing blood pressure measurement using tonometry, the sensor chip 101 is pressed against a measurement site 90 through which an artery 91 passes, as shown in FIG. 18. Accordingly, the artery 91 is pressed with an external pressure Po via the sensor chip 101, not such that the artery 91 is completely crushed, but rather such that only a portion 91a of the vascular wall of the artery 91 (the portion in the vicinity of a body surface 90a that opposes the sensor chip 101, which is hereinafter called the “outward-facing portion”) becomes flattened. In other words, the radius of curvature of the vascular wall of the outward-facing portion 91a is set to infinity. At this time, the sensor chip 101 is arranged such that the lengthwise direction thereof, that is to say the extending direction of the pressure sensor array 110 (X direction), intersects the artery 91. In this arrangement, based on the output of the pressure sensor elements 111, 111, . . . included in the pressure sensor array 110, an appropriate pressure sensor element 111 is selected from the pressure sensor array 110. Change in the internal pressure of the artery 91 is then measured based on the output from the selected pressure sensor element 111.
Specifically, as shown illustratively in FIG. 19(B), while maintaining equilibrium between the external pressure Po and a blood vessel internal pressure Pi, the internal pressure (blood pressure pulse wave) Pi pulsating against the external pressure Po is obtained. Note that as shown illustratively in FIG. 19(A), if a flat portion is not formed in the artery 91, the radius of curvature of the vascular wall (indicated by r) becomes relatively smaller in Po=Pi+T/r, which is the relational expression for the external pressure Po and the internal pressure Pi. For this reason, the external pressure Po and the blood vessel internal pressure Pi substantially do not match, and accurate measurement cannot be performed.